Computing and networking technology have transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. Many high speed data transmission networks rely on optical transceivers and similar devices for facilitating transmission and reception of digital data embodied in the form of optical signals over optical fibers. Optical networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to as grandiose as the backbone of the Internet.
Typically, data transmission in such networks is implemented by way of an optical transmitter (also referred to as an electro-optic transducer), such as a laser or Light Emitting Diode (LED). The laser emits light when current is passed through it, the intensity of the emitted light being a function of the current magnitude passed through the laser. Information is conveyed over an optical fiber by transmitting different optical intensities onto the fiber.
The laser has strong temperature dependencies that can seriously affect performance, depending on the application. For example, in Dense Wavelength Division Multiplexed (DWDM) laser applications, different optical channels are transmitted simultaneously, each optical channel having a tight frequency range that the corresponding optical signal should stay within. Any variance outside of the frequency range could cause inter-signal interference, seriously increasing the error rate of the transmission. Thus, in DWDM laser applications, it is critical that the laser's transmitted frequency be tightly controlled. Nevertheless, the frequency characteristics of a laser are heavily temperature-dependent. More specifically, the frequency characteristics of the optical emissions from the PN junction of the laser are heavily dependent on temperature. Thus, in DWDM laser applications, there is tight control of the temperature of the electro-optic transducer. Although DWDM has been discussed here, there are a wide variety of applications in which it may be desirable to accurately control the temperature of the emitting PN junction of the laser.
The temperature control of the laser typically relies on a temperature feedback system. Specifically, a temperature sensor is provided in proximity to the electro-optic transducer. Depending on the sensed temperature, a thermoelectric cooler (TEC) then heats or cools the temperature sensor as appropriate until the temperature sensor detects a temperature within an acceptable temperature range. The aim here is that by tightly controlling the temperature of the temperature sensor, the temperature of the proximate laser will also be tightly controlled.
Typically, the TEC is controlled by a TEC driver that is placed outside the subassembly housing containing the TEC, laser, and sensor. Accordingly, electrical connections must be made from the external TEC driver to the TEC inside the optical subassembly housing.
Such connections, while providing the necessary electrical signal to the TEC, have many disadvantages. For example, these connections require multiple high current, low impedance connections between the TEC and the TEC driver. Further, there are often Electro-Magnetic Interference (EMI) problems associated with these connections as the TEC driver output is often noisy. Finally, in many applications a large filtering component is required at the output of the TEC driver to control for signal impurities.